It is desirable to increase a perspective feeling of the image on the monitor, by changing an image distance of the virtual image. For example, in the case that an image information relating to a forward landscape will be displayed as the virtual image, wherein the image information is overlapped to the forward landscape, the image can be recognized without uncomfortable feeling and a more realistic feeling can be obtained when the image distance of the virtual image is changed depending on a distance between the observing person and the landscape in the display device.
More exactly, when the distance between the observing person and the landscape in the monitor device is longer, the distance between the observing person and the virtual image to be displayed on the monitor device will be likewise made longer, so that the more realistic feeling can be obtained.
An advantage of displaying the virtual image as an additional image is more clearly understood when compared with an ordinary stereoscopic television, with which the perspective feeling can be also obtained.
The ordinary television has an outer frame outside of a display area, and therefore eyes of the observing person are likely focusing on the outer frame. In this situation, when a distance of a display image is largely changed, it may give an uncomfortable feeling to the observing person.
Accordingly, the stereoscopic television may display such an image, which provides a less perspective feeling. On the other hand, when an additional image is displayed as a virtual image, the image distance can be dynamically changed, because the outer frame of a display device can be little recognized.
In a conventional virtual image display apparatus, an image on a display device is magnified by a concave mirror or a lens and displayed as a virtual image, wherein optical devices are mechanically moved to change the image distance.
In another conventional virtual image display apparatus, such a system having an optical projection system and a field lens is disclosed, wherein there are provided with a pair of optical projection systems and images at exit pupils of the projection lenses are formed at right and left eyes of an observing person, namely light rays from the optical projection systems are focused at the right and left eyes.
For example, those conventional apparatus is disclosed in Japanese Patent Publication (Un-examined) H5-147456 or 2001-356298.
In the case that the image distance is changed by mechanically moving the optical devices, a large-sized screen is necessary or the optical system itself becomes larger when the image distance is made longer.
In the case that the conventional virtual image display apparatus is used to an automobile and a front windshield is used as a translucent reflecting means, curvatures of the windshield largely vary and as a result the image on the windshield is largely deformed.
Furthermore, in the conventional apparatus having a pair of optical projection systems and a field lens, wherein the light rays from the respective optical projection systems are collected at the left and right eyes by the use of the field lens, an optical axis of the pair of projection systems is displaced from an optical axis of the field lens, causing an aberration. As a result, the light rays from the whole area of the lens may not be collected at one observing point, so that the whole image can not be recognized by the observing person, causing a hiatus of the image. Furthermore, when the displayed image is made larger, the field lens must be made larger, and thereby the aberration becomes inevitably much larger, decreasing the visibility.
Even if the lens without aberration was realized, the hiatus of the image may easily happen when the observing points, (eyes) of the observing person are moved even by a small distance, because a focusing point is within a small area. Accordingly, even when a position of the observing points is detected to control the light rays, it is difficult to obtain a high visibility.
It is, therefore, necessary to make larger a diameter of the light flux at the focus point, by making larger an exit pupil of a projection lens. When the exit pupil is made larger, however, a distance between the projection lens and the field lens, namely projection distance, becomes larger, resulting in a large sized optical system.
As above, there are many problems in the conventional apparatuses, namely
an optical system becomes larger in case that the some of the optical devices of the system is mechanically moved,
a visibility is decreased (a hiatus of image) because of the aberration of lens, and
an optical system becomes larger in case that an exit pupil is made larger.